German document DE 10 2005 061 879 A1 discusses a sheathed-element glow plug for an internal combustion engine, in which a pressure-measuring unit is accommodated in an housing. The sheathed-element glow plug further includes a heating element which is displaceable with respect to the housing and cooperates at least indirectly with the pressure sensor so as to enable a measurement of the pressure in the combustion chamber of the internal combustion engine. For this purpose, the heating element is acted upon with the pressure that prevails in the combustion chamber, and, via a supporting tube and a force transmission sleeve, a force is exerted upon the pressure sensor. The pressure sensor may be executed as a piezoelectric element, which is picked off via measuring lines.
Furthermore, the sheathed-element glow plug has an end on the combustion chamber side which is exposed to the conditions, especially the temperature, that prevail in the combustion chamber. At the end on the combustion chamber side, the sheathed-element glow plug has a sealing chamber in which a flexible diaphragm is accommodated. Between the diaphragm and the heating element there is additionally provided a sealing agent which is connected ahead of the flexible diaphragm. The sealing agent protects the sealing chamber from contamination which, after a certain operating duration, would reduce the accuracy of the measurement of the pressure in the combustion chamber. In this context, the sealing agent may be configured to be temperature-resistant and flowable. Alternatively, the sealing agent may be in the form of a powder or a paste.
German patent document DE 10 2004 012 673 A1 discusses a sheathed-element glow plug for an internal combustion engine, which includes a spark-plug shell, a glow element and an integrated pressure sensor. Between the glow element and the spark-plug shell a relative motion is possible. In operation, the glow element is acted upon by a combustion chamber pressure, whereby the glow element carries out a relative motion with respect to the spark-plug shell. In the light of the extent of the relative motion, the pressure sensor ascertains the magnitude of the pressure that prevails in the combustion chamber. Between an inner surface of the spark-plug shell and the glow element there is an elastic element which seals the space between the spark-plug shell and the glow element. The elastic element is configured as an annular element or a spring element. An alignment of the elastic element takes place via a supporting part, a wraparound ring applied inside the housing or welding seams.
The flexible diaphragm applied between the glow element as force transmission body and the spark-plug shell has a thermal shock applied to it, by the high temperatures prevailing in the combustion chamber of the internal combustion engine, whereby its elastic behavior changes, and because of that, the measuring result becomes corrupted. Furthermore, the thermal shock represents a thermomechanical stress of the flexible diaphragm, in response to which high local and temporal temperature gradients occur in the diaphragm. Thermal expansions and stresses go along with the high temperature gradients, which reduce the service life of the flexible diaphragm.